Industrially, fluorinated hydrocarbons are generally prepared by a halogen exchange reaction of halocarbons, such as chlorohydrocarbons or bromohydrocarbons--most usually chlorohydrocarbons, with inorganic fluorine compounds such as alkali metal fluorides, antimony fluorides, silver fluorides and mercury fluorides or with hydrogen fluoride in the presence of a catalyst.
As recognized in the art, the ease with which a halogen exchange reaction occurs depends primarily upon the particular manner in which the halogen atom to be exchanged for fluorine is attached to the carbon atom of the halocarbon. Thus, Hudlicky in Chemistry of Organic Fluoride Compounds, MacMillan Co., New York, NY (1962), pg. 98, points out the difficulty of converting the --CH.sub.2 Cl group to the --CH.sub.2 F group under various reaction conditions. Similarly, Sheppard and Sharts in Organic Fluorine Chemistry, W. A. Benjamin, Inc., New York, NY (1969), pp. 76-77, also teach that the Cl radical in the --CH.sub.2 Cl group is of very low reactivity and that, when reacted with fluorinating agents, rarely undergoes reaction to provide --CH.sub.2 F group.
Furthermore, it is known that fluoro-substitution on a carbon atom adjacent to the --CH.sub.2 Cl group reduces the reactivity of the Cl radical of the --CH.sub.2 Cl even more. Thus, with the above two unfavorable conditions, to prepare CF.sub.3 CH.sub.2 F by the vapor-phase reaction of CF.sub.3 CH.sub.2 Cl with HF requires rather drastic reaction conditions. Bell in U.S. Pat. No. 4,129,603 describes (column 3, lines 55-68) the reaction of CF.sub.3 CH.sub.2 Cl with HF with an HF to CF.sub.3 CH.sub.2 Cl ratio of 4:1, a reaction temperature of 350.degree. C. and a contact time of 7 seconds continually over a period of 55 hours using a Cr.sub.2 O.sub.3 catalyst pretreated with HF. The product stream contained 18.2% CF.sub.3 CH.sub.2 F, 80% unreacted CH.sub.3 CH.sub.2 Cl, 1.65% of a mixture of CF.sub.3 CHF.sub.2 and CF.sub.3 CH.sub.3 and 0.12% CF.sub.2 .dbd.CHCl. What is not disclosed in the above-cited Bell patent is that in the reaction of CF.sub.3 CH.sub.2 Cl with HF in the presence of a Cr (III) catalyst at elevated temperatures, with time, the ability of the Cr (III) catalyst to catalyze the halogen exchange reaction is drastically reduced, such that the process becomes useless as an industrial process in a very short time. Depending upon the reaction conditions, the catalyst may become completely inactive in a period of from about 150 to 300 hours.
It is reasonable to speculate that this drastic loss of catalyst activity is due to the rather high temperatures required to make this particular halogen exchange reaction proceed to a reasonable extent. Such elevated temperatures will increase dehydrohalogenation reactions, such as CF.sub.3 CH.sub.2 Cl to CF.sub.2 .dbd.CHCl+HF, and even of the desired product, CF.sub.3 CH.sub.2 F to CF.sub.2 .dbd.CHF+HF. These olefinic compounds can react, particularly on the catalyst surface, to form polymers, carbon or coke deposits which may make the catalyst surfaces unavailable for catalytic activity. Whatever the mechanism, in the reaction of CF.sub.3 CH.sub.2 Cl and HF over a Cr (III) catalyst at elevated temperatures, the catalyst will rapidly lose activity over a period of time; the higher the reaction temperature the faster will be the loss of activity.
Osaka et al. in British Patent 2,030,981 have recognized the above-discussed loss of activity of the chromium (III) catalyst in the halogen exchange reaction at high temperatures between CF.sub.3 CH.sub.2 Cl and HF. The patentees suggested introduction into the feed stream of from 0.002 to 0.05 mol O.sub.2 per mol of CF.sub.3 CH.sub.2 Cl to prolong the activity of the Cr (III) catalyst. While cofeeding of O.sub.2 with CF.sub.3 CH.sub.2 Cl prolongs the catalytic activity of Cr (III) catalysts, such modification cannot be used advantageously in an industrial process because of the additional problems it introduces. Among them are greatly increased formation of water in the reaction which increases the difficulty in isolating the desired product, and which greatly increases the corrosion in the condensed phase due to hydrofluoric and hydrochloric acids generated therefrom. Reaction of oxygen with various components of the reaction stream also introduces many undesirable oxygenated by-products.
It is, therefore, an object of the present invention to provide an improved process for manufacturing 1,1,1,2-tetrafluoroethane by the vapor phase reaction of 2-chloro-1,1,1-trifluoroethane with hydrogen fluoride in the presence of an inorganic chromium (III) catalyst. It is a further object of the present invention to provide a process for manufacturing 1,1,1,2-tetrafluoroethane by the vapor phase reaction of 2-chloro-1,1,1-trifluoroethane with hydrogen fluoride in the presence of an inorganic chromium (III) catalyst wherein the activity of the chromium (III) catalyst is maintained.